The history of and uses of absorption cycles in general, and in particular the application of NaOH and/or KOH to absorption cycles, is summarized briefly in copending application Ser. No. 06/658,540 filed 10/09/84 by Donald C. Erickson, now abandoned, which is incorporated by reference.
U.S. Pat. No. 2,795,115 recites the use of "solutions of one or more of the soluble basic hydroxides: for example, sodium and potassium hydroxides" in an air-cooled absorption refrigeration cycle. That patent discloses that "using the ambient air for cooling is shown to be practical only in the case of sodium hydroxide or other basic hydroxide solutions of similar absorption characteristics to the sodium hydroxide solution displayed in FIG. 3". FIG. 3 illustrates a substance which at a water vapor pressure of 6 mm Hg (40.degree. F. evaporator) does not precipitate to solid until the temperature is above 320.degree. F.
Other U.S. Patents which recite the use of aqueous NaOH in an absorption cycle to produce refrigeration include U.S. Pat. Nos. 4,151,721 and 2,497,819.
In the report "Candidate Chemical Systems for Air Cooled, Solar Powered, Absorption Air Conditioner Design", SAN-1587-2, by W. J. Biermann of Carrier Corp. (April 1978), the possibility of the use of "the very soluble alkali metal (except lithium) hydroxides" as absorbents is considered. Thermodynamic data somewhat different from that of FIG. 3 referred to above is presented, and ascribed to NaOH. The report concludes that, "Its thermodynamic promise is marginal and the generally corrosive properties of strong caustic soda lead to a decision to reject."
The basic hydroxides were experimented with and temporarily put into actual use as absorbents in the late 1800s. In the mid 1950's they were suggested for air-cooled refrigeration use, and more recently for heating and heat storage. Yet as of 1978 they are not in commercial use and recommended not to be used.
It has now been discovered that there are several good reasons why the previously disclosed solutions are not in use, and that those problems are only avoided by the solutions newly disclosed herein, and containing CsOH.
Air cooled air conditioning requires that an absorption heat pump absorb heat at 4.degree. to 8.degree. C. and reject the heat at 45.degree. to 60.degree. C. Hot water heating and residential space heating require similar conditions. Building or district heating requires even hotter delivery temperatures, e.g., 65.degree. to 95.degree. C. All of these applications are beyond the capability of existing absorbents, as typified by LiBr.
The FIGURE depicts the approximate solubility limits of two known absorbents: LiBr and NaOH. As shown, for a 6 mm Hg water vapor pressure (corresponding to a 5.degree. C. evaporator temperature) NaOH solidifies at 44.degree. C. and LiBr at 53.degree. C. KOH has approximately the same solubility limit as LiBr at this pressure. After allowing a prudent operating margin of about 15.degree. C. from crystallization, it can be seen that these absorbents are totally unsuited for the applications outlined above. Even should NaOH be operated with a 10.degree. C. evaporator, which increases the water vapor pressure to 9.2 mm Hg, and increases the solubility limit to 84.degree. C., there still remain problems with NaOH. In addition to the corrosion problem, there is the startup and shutdown problem. Starting from any NaOH concentration which takes advantage of its extended solubility limit beyond LiBr, and cooling that concentration to ambient temperature, the crystallization (i.e., solid precipitation) temperature is encountered first. Should this happen, the absorbent must be remelted prior to subsequent startup, which can be extremely difficult.
The application cited above discloses that certain mixture proportions of NaOH and KOH will both suppress the "hump" of NaOH sufficiently to allow 5.degree. C. evaporator operation without crystallizing at the hump, and also extend the overall solubility limit beyond that of NaOH neglecting the hump. However, when sufficient NaOH is retained in the mixture to achieve this extension of solubility limit, a hump will still be present sufficiently so as to cause solidification upon cooling to ambient. The NaOH-KOH solubility limit curve of the FIGURE, representing approximately 75% NaOH and 25% KOH, illustrates this problem.
In addition to corrosiveness, there is still another problem with absorbent mixtures consisting primarily of NaOH. This is the limited solubility of sodium carbonates and bicarbonates. Upon exposure to air, e.g., due to maintenance or a leak, the absorbent turns cloudy due to carbonate precipitation. This could clog nozzles, valves, and the like.
What is needed is an absorbent which has a solubility limit comparable to or even greater than that of NaOH, to enable it to serve the high lift applications from low temperature described above, yet still be able to cool to ambient temperature without crystallization, to accept corrosion inhibitors without serious degradation, and be exposed to trace amounts of CO.sub.2 without solids precipitation. No previously identified aqueous absorbent has satisfied these requirements.